1. Field of the Invention
This invention relates to a wireless communication system making indoor wireless communication between a wireless base station unit and a wireless terminal unit.
2. Description of the Related Art
A wireless communication system making indoor wireless communication between a wireless base station unit and a wireless terminal unit in an indoor space as shown in FIG. 14 has been known. A plurality of wireless base station units 2 are arranged on a ceiling of an indoor space 1 where a wireless LAN is to be worked, and a LAN cable 3 and a power supply cable 4 are connected to the wireless base station units 2 from the outside.
Each of the wireless base station units 2 makes wireless communication with a wireless terminal unit 6 located in a cell 5 which is regarded as a wireless communication range. For this reason, the cells 5 need to be arranged to cover an entire floor surface of the indoor space 1. The wireless base station units 2 are arranged such that the cell overlaps the others to some extent in view of safety. For example, a radius of the cell 5 on the floor formed by the wireless base station unit 2 is set at approximately 20 m. Therefore, if the floor surface is 50 m square, at least four wireless base station units 2 need to be arranged.
In addition, a wireless communication system disclosed in Jpn. Pat. Appln. KOKAI Publication No. 06-188821 is also known. In this system, a base station comprises antenna transmitting and receiving means for making wireless communication with a terminal station located in a service area and coaxial cable transmitting and receiving means having a comparatively large output to make wireless communication with a terminal station having a blind area that a radio wave irradiated from the base station does not reach effectively. Inside the blind area, an antenna or a leaky coaxial cable is arranged in accordance with the shape of the blind area. The antenna or leaky coaxial cable and the feeder transmitting and receiving means of the base station are connected to each other by a feeder having a predetermined attenuation constant and a predetermined length. An output level of the radio wave irradiated from the antenna or leaky coaxial cable is therefore set not to exceed a level of the radio wave from the antenna transmitting and receiving means of the base station.
In a cell division scheme such as the former case, however, more wireless base station units need to be arranged to remove the blind areas caused by furniture and fixtures, in the indoor environment where there are a number of furniture and fixtures as disturbances of the radio wave propagation. For this reason, the total number of employed wireless base station units is increased, the number of installation works of the wireless base station units is increased in accordance with the increased number thereof, and the entire installation costs are increased.
Moreover, if the number of arranged wireless base station units is increased, incoming waves from the wireless base station units cause interference that carrier waves erase each other, at the receiving antenna. For this reason, no effect can be obtained even if a modulation and demodulation scheme resistant to the multipath interference such as the orthogonal frequency division multiplex modulation (hereinafter referred to as OFDM) scheme is applied. Therefore, a problem arises that even if all the wireless base station units simultaneously emit the radio waves, broadcast transmission cannot be executed for all the wireless terminal units. In addition, transmission can be executed to the wireless base station units one after one by time division. In this case, however, much time is spent to complete the entire information transmission.
In the latter scheme, leaky coaxial cables are further arranged in the blind area besides the wireless LAN system including general base stations, and wireless installations serving as the feeder transmitting and receiving means exclusive for the leaky coaxial cables. For this reason, the system becomes large in size and the total installation costs are increased.
The object of the present invention is to provide a wireless communication system which can prevent generation of the indoor blind areas, and which can implement high-speed broadcast transmission causing no intersymbol interference at small costs.